KVM guest sometimes failed to boot because of kernel stack overflow if KPTI is enabled on a hisilicon ARM64 platform.

[xuwei5@hisilicon.com (Wei Xu)]

Hi Will,

On 2018/6/21 10:18, Will Deacon wrote:
> On Thu, Jun 21, 2018 at 09:38:53AM +0100, James Morse wrote:
>> On 20/06/18 17:25, Wei Xu wrote:
>>>     [    0.042421] Insufficient stack space to handle exception!
>>>     [    0.042423] ESR: 0x96000046 -- DABT (current EL)
>>>     [    0.043730] FAR: 0xffff0000093a80e0
>>>     [    0.044714] Task stack: [0xffff0000093a8000..0xffff0000093ac000]
>>
>> This was a level 2 translation fault on a write, to an address that is within
>> the stack....
>>
>>
>>>     [    0.051113] IRQ stack: [0xffff000008000000..0xffff000008004000]
>>>     [    0.057610] Overflow stack: [0xffff80003efce2f0..0xffff80003efcf2f0]
>>>     [    0.064003] CPU: 0 PID: 12 Comm: migration/0 Not tainted
>>> 4.17.0-45865-g2b31fe7-dirty #10
>>>     [    0.072201] Hardware name: linux,dummy-virt (DT)
>>
>>>     [    0.076797] pstate: 604003c5 (nZCv DAIF +PAN -UAO)
>>>     [    0.081727] pc : el1_sync+0x0/0xb0
>>
>> ... from the vectors.
>>
>>
>>>     [    0.085217] lr : kpti_install_ng_mappings+0x120/0x214
>>
>> What I think is happening is: we come out of the kpti idmap with the stack
>> unmapped. Shortly after we access the stack, which faults. el1_sync faults as
>> well when it tries to push the registers to the stack, and we keep going until
>> we overflow the stack.
>>
>> I can't reproduce this with kvmtool or qemu in the model.
> 
> Hmm, one thing that occurs to me is that the kpti_install_ng_mappings()
> code leaves the nG bit set in table entries, which is actually IGNORED in
> the architecture.
> 
> Wei -- does the diff below help at all? Make sure you disable CONFIG_KASAN,
> otherwise your kernel will take an age to boot.

Yes, amazing! This patch resolved the issue.
I have tested 50 times and can not reproduce the issue any more.
Could you please tell more why this patch works?
Thanks!

Best Regards,
Wei

> 
> Will
> 
> --->8
> 
> diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
> index 5f9a73a4452c..70d9e98467ca 100644
> --- a/arch/arm64/mm/proc.S
> +++ b/arch/arm64/mm/proc.S
> @@ -272,8 +272,8 @@ ENTRY(idmap_kpti_install_ng_mappings)
>  	add	end_pgdp, cur_pgdp, #(PTRS_PER_PGD * 8)
>  do_pgd:	__idmap_kpti_get_pgtable_ent	pgd
>  	tbnz	pgd, #1, walk_puds
> -next_pgd:
>  	__idmap_kpti_put_pgtable_ent_ng	pgd
> +next_pgd:
>  skip_pgd:
>  	add	cur_pgdp, cur_pgdp, #8
>  	cmp	cur_pgdp, end_pgdp
> @@ -302,8 +302,8 @@ walk_puds:
>  	add	end_pudp, cur_pudp, #(PTRS_PER_PUD * 8)
>  do_pud:	__idmap_kpti_get_pgtable_ent	pud
>  	tbnz	pud, #1, walk_pmds
> -next_pud:
>  	__idmap_kpti_put_pgtable_ent_ng	pud
> +next_pud:
>  skip_pud:
>  	add	cur_pudp, cur_pudp, 8
>  	cmp	cur_pudp, end_pudp
> @@ -323,8 +323,8 @@ walk_pmds:
>  	add	end_pmdp, cur_pmdp, #(PTRS_PER_PMD * 8)
>  do_pmd:	__idmap_kpti_get_pgtable_ent	pmd
>  	tbnz	pmd, #1, walk_ptes
> -next_pmd:
>  	__idmap_kpti_put_pgtable_ent_ng	pmd
> +next_pmd:
>  skip_pmd:
>  	add	cur_pmdp, cur_pmdp, #8
>  	cmp	cur_pmdp, end_pmdp
> 
> .
>